EXOGENOUS STEM — PITH. 53 



posed of cellular tissue, which is developed in an upward direction, the 

 cells diminishing in size towards the circumference, and being often 

 hexagonal. In the young plant it occupies a large portion of the stem, 

 and sends cellular processes outwards at regular intervals to join the 

 medullary rays (figs. 11 1, 11 2 y). The pith has at first a greenish hue, 

 and is full of fluid, but in process of time it becomes pale-coloured, 

 dry, and full of air. These changes take place first in the central cells. 

 Sometimes the pith is broken up into cavities, which have a regular 

 arrangement, as in the Walnut, Jessamine, and Cecropia peltata ; it is 

 then called discoid or disciform (diaxo.^, a disc, from the circular parti- 

 tions). At other times, by the rapid growth of the outer part of the 

 stem, the pith is ruptured irregularly, and forms large cavities as in 

 the fistular Stem of Umbelliferous plants. Circumscribed cavities in 

 the internal cellular portions of stems are by no means unfrequent, 

 arising either from rupture or absorption of the cells. In some rare 

 instances vessels occur in pith, as in Elder, Pitcher-plant, and Ferula ; 

 and occasionally its cells are marked by pores indicating the formation 

 of secondary deposits. The exteat of pith varies in different plants, 

 and in difierent parts of the same plant. In Ebony it is small, while 

 in the Elder it is large. In the Shola plant, jEschynomene aspera, 

 the interior of the stem is almost entirely composed of cellular tissue 

 or pith ; from this a kind of rice-paper is made, and light hats. The 

 same kind of tissue occurs in the Papyrus of the Nile. Large pith is 

 also seen in Fatsia papyrifera, or Chinese rice-paper plant. When the 

 woody circle of the first year is completed, the pith remains stationary 

 as regards its size, retaining more or less its dimensions, even in old 

 trunks, and never becoming obliterated. 



The Medtjllary Sheath is the fibro-vascular layer immediately 

 surrounding the pith. It forms the inner layer of the vascular bundle 

 of the first year (fig. 114 t), and consists chiefly of true spiral vessels, 

 which continue to exercise their functions during the life of the plant, 

 and which extend into the leaves. With the spiral vessels there are a 

 few woody fibres intermingled. The processes from the pith are pro- 

 longed into the medullary rays between the vessels of the sheath. 



Woody Layers. — During the first year the vascular circle con- 

 sists of an internal layer of spiral vessels forming the medullary sheath, 

 and external bundles of pitted and ligneous vessels. In subsequent 

 years the layer of spiral vessels is not repeated, but concentric zones 

 of pitted vessels (fig. 116 ter) and pleurenchyma are formed, consti- 

 tuting what are commonly called the woody circles of trees. The 

 vascular bundles, from their, mode of development in an indefinite 

 manner externally, have been called Exogenous; and, for the same 

 reason, Schleiden has denominated them Indefinite. Exogenous plants 

 have sometimes received the name of Cydogens (x-jxXog, a circle), 

 in consequence of exhibiting concentric circles in their stems. On a 



